This.
Coming from a person who has a loved one on the spectrum: kudos to you for recognizing and adapting to your DS' needs. Too often, parents just tell their kids to "stop being difficult," and it's heartbreaking.
IMO, this is not a hard thing for CMs to accommodate. Worst case: you're asking the serve to maybe pick an alternate path to serve a few tables for 30-60 minutes of their entire day so your son can stand. Best case: they have a chair that is maybe off-theme, but fits his needs, that they can put in a place that doesn't make him uncomfortable or them inconvenienced. So, I would insist on eating where you want and putting it on the CMs to figure it out. It can be done in a polite and respectful, but still assertive, way. In my experience, WDW is almost always very very good about A) accommodating, and B) not making you feel bad about needing an accommodation.
If you do figure out where you want to eat in advance, it might be helpful to reach out to guest services or the restaurant itself and let them know what you'll be asking of them (I'd probably pick guest services because, in my experience, they tend to be the best trained in these matters). It's really not that hard to bring a stool/bench over from backstage or somewhere else if they know a couple hours in advance.
I do think we caretakers of ND folks need to be a little more assertive/empowered, and less of the "this is our burden to bear" mindset, at times. Your DS needs (NEEDS!) this, he and you are entitled to eat where you want just like everyone else, you should not have to be going on message boards to find out where you all can eat with the default arrangements, and, at the end of the day, this is such a very minor ask from you.
And
@jimmymc - that's either an incredible memory you have, you live at WDW, or you're a saint for going to all of these parks to research for OP. I'm genuinely curious how you came up with such a detailed list because those are the only explanations I can come up with lol.